Cluster spacecraft reveals new insights into the inner workings of the Earth’s natural particle accelerator

Shocks
are abundant throughout the Universe, and this image shows multiple
shocks which have been detected from numerous astronomical sources.
(See full version and copyright below)

A new study performed by the Swedish
Institute of Space Physics in Uppsala, in collaboration with the
University of Sheffield and other groups, uses data from the European
Space Agency’s Cluster spacecraft to reveal new insights into the inner
workings of the bow shock when it becomes non-stationary and its
structure starts to break down.

The sun continuously ejects a stream of charged particles travelling at
supersonic speeds from 300 to over 1000 km/s. When this “solar wind”
plasma reaches Earth, it encounters an obstacle which is the Earth’s
protective magnetic shield, creating a shockwave when the solar wind
decelerates. Such shockwaves are found throughout the Universe around
stars, supernova remnants, comets and planets. They are particularly
important because they are efficient particle accelerators, and may be
the origin of some of the most energetic particles in the Universe.

The shockwave upstream of the Earth is known as the bow shock, named
after the shape of waves breaking in front of the bow of a ship as it
moves through water. This shockwave differs to others such as that
formed around an aircraft breaking the sound barrier since collisions
between particles do not transfer energy. Since the plasma density is
very low, particle collisions are extremely rare and negligible. Such
types of shocks are therefore called collisionless. The interaction
between particles and electromagnetic fields transfers energy in the
absence of collisions.

Under certain conditions, the bow shock cannot redistribute the
required amount of energy to maintain its structure and it becomes
non-stationary, initiating a wave-breaking process.

The mechanism which initiates non-stationarity of the Earth’s bow shock
was unclear until now due to the lack of closely separated spacecraft
providing measurements at the right time and place.

"These key minutes of data have been years in the planning, and started
with the proposal of a carefully planned close-separation campaign
designed to place two of the four Cluster spacecraft less than seven
kilometres apart. Thanks to this we were able to make this unique and
important measurement," says lead author Andrew Dimmock of the Swedish
Institute of Space Physics in Uppsala, Sweden. "This result advances
our understanding of one of nature’s most efficient particle
accelerators," adds Dimmock.

Launched in 2000, the European Space Agency’s Cluster mission is
comprised of four spacecraft in orbit around Earth and was the first to
probe the Earth’s magnetic environment in three dimensions.

"The Cluster mission has been a cornerstone for space physics, and has
led to many new discoveries related to the Earth’s bow shock," says
Dimmock. "In this case, the unprecedented ability to study its
structure on scales of only a few kilometres means that for the first
time we could resolve the key intricate spatial structures which are so
important to dictating its large-scale behaviour".

The researchers found that although there are two physical descriptions
of non-stationarity, one is valid for the conditions at the Earth’s bow
shock. Such a description involves the presence of an electron scale
structure embedded within a thin region of the shock that undergoes the
most dramatic change, known as the ramp.

"This result demonstrates the significance of the Cluster mission, and
its ability to still deliver important and novel scientific results",
says ESA project scientist Philippe Escoubet. Co-author Michael
Balikhin also adds that "such work was only possible due to the
hard-working engineers, scientists and operation staff in conjunction
with the active Cluster community".

It comes as no surprise that the Cluster science and operations team
were recently honoured by the Royal Astronomical Society by a
well-deserved group achievement award.

Andrew Dimmock and co-authors published these results in the journal
Science Advances, in an article entitled “Direct evidence of
nonstationary collisionless shocks in space plasmas”. The article is
freely available (open access) from the Science Advances webpage: http://advances.sciencemag.org/

Illustration and copyright:
Shocks are abundant throughout the Universe, and this image shows
multiple shocks which have been detected from numerous astronomical
sources. Such shocks are formed when fast supersonic plasma suddenly
encounters an obstacle such as a planetary magnetic field (such as at
Earth), or a much slower flowing plasma.

The Swedish Institute of Space Physics (IRF) is a governmental research institute which conducts research and postgraduate education in atmospheric physics, space physics and space technology. Measurements are made in the atmosphere, ionosphere, magnetosphere and around other planets with the help of ground-based equipment (including radar), stratospheric balloons and satellites. IRF was established (as Kiruna Geophysical Observatory) in 1957 and its first satellite instrument was launched in 1968. The head office is in Kiruna (geographic coordinates 67.84° N, 20.41° E) and IRF also has offices in Umeå, Uppsala and Lund.